I. Field of the Invention
The method and apparatus for forming articles containing controlled amounts of binder of this invention is found in U.S. Patent Office Manual of Classification, Class No. 264, Subclass 41, which relates to pore forming in situ.
II. Description of the Prior Art
When forming articles containing controlled amounts of binder, it is customary to only include initially the desired amount of binder. Occasionally, this may not be suitable manner for making some articles.
For instance, simple ceramic articles are usually formed by dry pressing, casting, extruding, or soft mud molding. Once the desired shape is formed, this unfired (green) ware is usually dried and fired to a temperature which will produce the desired porosity and strength in the ceramic piece. With dry pressing, it is difficult to obtain uniform density throughout the formed piece and intricate shapes cannot be formed without machining the unfired or fired ware. Extrusion and soft mud methods are even more limited in the shapes which can be formed.
Casting utilizes a slurry of ceramic and water which can be poured into a mold of the desired shape. Intricate shapes can be formed by casting, but thick-walled articles require a great amount of time to dry before firing and significant density variations can exist. With any of the methods described, it is difficult to control precise dimensions and density of the fired article.
More recent prior art methods for forming more complex-shaped articles are found in U.S. Pat. No. 3,351,688 to Kingery which issued on Nov. 7, 1967, and is entitled PROCESS OF CASTING REFRACTORY MATERIALS, and U.S. Pat. No. 3,416,905 to Waugh which issued on Dec. 17, 1968, and is entitled PROCESS FOR MANUFACTURE OF POROUS ABRASIVE ARTICLES.
U.S. Pat. No. 3,351,688 teaches a method of casting refractory materials by forming the articles in a green state from a slurry of refractory materials, a meltable binder, and a suspending agent. The piece is formed in a liquid state, allowed to solidify, and is then heated to vaporize the binder. The piece is then fired in its final configuration to a desired density. U.S. Pat. No. 3,416,905 teaches a similar process except that the fluid mixture formed comprises abrasive particles, glass frit, and a suspending agent in a meltable binder. The mixture is poured or injection molded onto an article, is allowed to solidify, is heated to vaporize the binder, and subsequently fired forming a porous abrasive article. The glass frit vitreously bonds the abrasive material into the final product.
In use of a meltable binder in forming articles, it has been the practice of those skilled in the art to heat the article being formed to a temperature sufficient to drive the binder from the article as vapor.
Although these uses of a meltable binder represent an improvement over the prior art, particularly for forming intricately shaped articles, vaporization of the binder from the articles has limited the size of the article which may thusly be manufactured. If the article wall thickness is greater than about one-half of an inch, cracking, scaling, and other matrix imperfections can result in the structure during vaporization of the binder. Further, if it is desired to closely control the amount of binder in the article, vaporization of the binder does not allow a regulated discharge of the binder.
Also, the internal pressure caused by the vaporization process can be great enough to produce laminations or internal voids in the article. Thus, if the structure being formed by these prior art methods requires certain porosity and density characteristics and close control of binder amounts present, e.g., thick-walled refractories, the structure thus formed may not be satisfactory.